1. Field of the Invention
This invention relates to compositions, and the methods of preparing the compositions, for stabilizing chlorinated water to sunlight decomposition. In particular, the invention includes a monoalkali metal cyanurate slurry and methods for preparing the slurry and a dry solid.
2. Description of the Related Art
For disinfection and biocontrol, chlorine is introduced to many different bodies of water that are exposed to direct sunlight. Examples include outdoor swimming pools, cooling towers, ornamental fountains and wastewater lagoons. There are a variety of ways of chlorinating these waters: sparging with gaseous Cl2; using a chemical delivery pump for liquid sodium hypochlorite (NaOCl) solutions; via erosion feeder for solid, tableted chlorine-donating chemicals such as Ca(OCl)2 and trichlorotriazinetrione; by hand-broadcasting fast-dissolving solid chlorine donating chemicals such as sodium dichlorotriazinetrione and LiOCl; and by electrolytic oxidation of Cl− ion from NaCl introduced to the water system.
It is well known that, regardless of the way that water is chlorinated, if the water is exposed to strong sunlight the chlorine will quickly disappear because of ultra violet (UV) light-induced photolytic degradation. The half-life of chlorine under such conditions has been estimated to be less than 45 minutes. In the late 1950s, it was discovered that the stability of chlorine to UV degradation could be significantly improved if cyanuric acid was introduced to the water. Cyanuric acid has never been surpassed as a chlorine stabilizer, and today it is still being introduced into chlorinated water for UV stabilization.
Typically, outdoor swimming pools and ornamental fountains are treated with solid or granular forms of cyanuric acid, usually sold in 1 lb. pouches, which the user is instructed to add in the deep end of the pool when the pool is being re-opened for the summer season or being filled with water for the first time. Between 25 and 80 ppm of cyanuric acid in the pool water provides effective UV stabilization properties. Unfortunately, the low solubility of cyanuric acid coupled with the low surface area of the granules means that the cyanuric acid may take several days to dissolve. Lower water temperatures, which may occur when pools are filled for the first time, prolong this dissolution time even further.
The extended time that it takes for cyanuric acid to dissolve in pool water causes several problems. First, the localized high acid condition in the vicinity of the undissolved cyanuric acid can etch or discolor the plaster of newly constructed gunite pools and can cause wrinkling and discoloration of the vinyl of vinyl-lined pools. Second, the pool remains unstabilized to UV decomposition until all the cyanuric acid has dissolved, and biological control is difficult due to the rapid UV degradation of the disinfectant. Third, the aesthetic appearance of the pool is compromised for several days due to piles of undissolved cyanuric acid lying at the bottom of the pool or ornamental water feature. Fourth, the pool owner is unable to operate the pool vacuum during this time because the cyanuric acid would be sucked from the floor of the pool into the filter. The resultant localized low pH condition in the filter and associated equipment would damage or impair the filtration systems and piping.
It has been suggested that the use of monosodium cyanurate will overcome these problems because it has about twice the solubility of cyanuric acid and would be expected to dissolve faster. Compared to cyanuric acid, monosodium cyanurate has a neutral pH and thus does not damage vinyl liners nor etch or discolor the concrete of gunite pools or water features.
The existing methods of producing monosodium cyanurate and the existing products, however, have several disadvantages. In the first place, monosodium cyanurate is not commercially available, probably due to a high cost of production. Production of solid, dry monosodium cyanurate requires the addition of NaOH to a slurry of cyanuric acid, followed by solid-liquid separation, drying, granulation, and screening. U.S. Pat. No. 4,432,959 describes an alternative process for producing the powder form of monosodium cyanurate by mixing solid isocyanuric acid with solid sodium carbonate or bicarbonate in the presence of sufficient water to hydrate the raw material at room temperature. Upon increasing the temperature to 60-100° C., water is liberated from the hydrated raw materials to form the solid monohydrate of monosodium cyanurate. The solid was said to be useful as an active chlorine stabilizer in pool water; however, the method is inefficient in producing large volumes and is still very costly.
Another method of overcoming these high production costs was discussed in U.S. Pat. No. 6,207,177 to Jany. This patent discloses a stable, gel-like 20-40% w/w slurry of monosodium cyanurate monohydrate in water, having a viscosity of 50,000-500,000 mPa s and a pH of 6.5 to 8.5. The method of making the gel involved adding 50% sodium hydroxide solution to a 20-50% slurry of cyanuric acid in water. No solid-liquid separation step was involved. The gel-like properties of the slurry were attributed to the monosodium cyanurate monohydrate crystals having a needle-like appearance with length to diameter ratios between about 5:1 and 20:1. The crystals were described as having formed a tangled mass that held a significant amount of free moisture and gave the gel the appearance of soft-serve ice cream. The slurry was reported to have very fast dissolution in water, making it useful to treat swimming pools, spas, hot tubs, cooling towers, artificial ponds, lagoons, decorative fountains, and other bodies of chlorinated water exposed to direct sunlight. The '177 patent also reports that the gel-like slurry had a tendency to dewater over time as the solids settled to the bottom of the container and that it was possible to decant this liquid layer. In addition, the '177 patent also discloses a potassium cyanurate slurry that did not have the same gel-like properties as the sodium cyanurate slurry and in which the solids separated rapidly.
Although the use of a slurry of monosodium cyanurate would appear to solve the problem of the high cost of production of solid monosodium cyanurate, it results in other problems. The present inventors replicated Example 2 of the '177 patent and produced a gel-like slurry exactly as described therein. It was found, however, that both the freshly-prepared and water-decanted slurries were so thick and viscous that they were of no practical value. In the first place, the slurry was so thick and viscous that it could not be easily pumped out of a reaction vessel into a container. Second, even if the slurry could be put into a container, the end-user would not be able to pour it out of its container into the pool; the user would have to scrape the slurry out with a separate tool or device. Moreover, because the slurries are so heavy and massive, they would immediately sink to the bottom of the pool and remain in an undissolved clump.
The viscosity problems of slurries of monosodium cyanurate monohydrate are evidenced by a product called Instant Pool Water Conditioner by Natural Chemistry of Norwalk, Conn. The product, stated to be a 36% slurry of monosodium cyanurate monohydrate in water, is reported to be instantly dissolving, having a pH of 8, and is asserted to be less damaging to pool surfaces than cyanuric acid. A one-gallon (3.8 liter) sample of this material was purchased from a local pool store. Upon opening the bottle, a volume of approximately two liters of clear water was decanted into a beaker, leaving behind a thick mass of settled solids. Vigorous shaking with additional water managed to dislodge the topmost portion of the compact mass from the container. After about 10 minutes of shaking, it was observed that the remainder was compacted into a solid, hard mass about the size of a fist. This mass could not be removed from the container.
Thus, there is clearly a need for a monosodium cyanurate formulation and method of manufacture that do not suffer from the above-mentioned problems. An ideal slurry product should: (1) be pourable into and out of its container; (2) not settle or compact to a hard mass at the bottom of the container; (3) not readily dewater (separate) in the container; (4) dispense and dissolve readily in the water to which it is added; and (5) be inexpensive and easy to manufacture. The methods and composition of the invention address these needs.